You need to backup data, especially irreplaceable items and documents or files that would create a burden to replace, and these backups need to be done regularly. It is always best to have these backups on an external hard drive, a removable drive such as a usb drive or burning them to a cd or dvd. This will enable your files to be restored if your hard drive fails or becomes corrupted.
Teachers here at school may backup their documents to the school's server. Larger files such as instructional videos and pictures should be backed up to either a cd or dvd. It is the responsiblity of each teacher to regularly backup any data they deem critical.